Remove the configmodel for push notifications.

This feature was used to enable/disable push notifications using
Parse.com.  That service died a few years ago but this was never cleaned
up.  Cleaning it up as a part of DEPR-41 now that we've removed the code
that references this model.

cms/djangoapps/contentstore/migrations/0004_remove_push_notification_configmodel_table.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.22 on 2019-07-26 20:12
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('contentstore', '0003_remove_assets_page_flag'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='pushnotificationconfig',
+            name='changed_by',
+        ),
+        migrations.DeleteModel(
+            name='PushNotificationConfig',
+        ),
+    ]